In the manufacture of optical fiber, a glass preform rod is suspended vertically and moved into a furnace at a controlled rate. The preform softens in the furnace and a glass fiber (also referred to as an optical fiber) is drawn freely from the molten end of the preform rod by a capstan located at the base of a draw tower. Because the surface of the glass fiber is susceptible to damage caused by abrasion, it is necessary to coat the fiber immediately after it is drawn but before it comes into contact with any surface. Inasmuch as the application of a coating material must not damage the glass surface, the coating material is applied in a liquid state. Once applied, the coating material must solidify before the glass fiber reaches the capstan. This is typically accomplished within a brief time interval by photocuring--a process in which the liquid coating material is converted to a solid upon exposure to electromagnetic radiation, preferably ultraviolet (UV) light.
Because the fibers are thin and flexible, they are readily bent when subjected to mechanical stresses such as those encountered during handling or exposure to varying temperature environments. Such bends in the fiber frequently result in optical loss that is much greater than the intrinsic loss of the fiber itself, and it has been found desirable to protect the glass fiber against such bending. Accordingly, the coating material is required to cushion the glass fiber against bends and two layers of coating materials are typically applied to the drawn optical fiber. An inner (primary) coating, having a relatively low in situ equilibrium modulus, is applied directly to the glass fiber. The in situ modulus of the primary coating is the equilibrium modulus of the coating measured on the fiber. An outer (secondary) coating, having a relatively high modulus, surrounds the primary coating. Together, these coatings protect the inherently high tensile strength of the glass fiber as long as the primary coating remains bonded to the glass.
The interfacial bond between the fiber and the primary coating desirably is strong to prevent bond failure during fiber manufacturing and subsequent handling. Such failure can result in the formation of a "delamination" area which can adversely affect the optical performance of the fiber. A delaminated area is characterized by a gap at the interface of the fiber and the primary coating. The gap alters the mechanical properties at the point of delamination and may cause fiber transmission losses. Even if the optical performance is substantially not affected, the delamination can result in negative customer perception and is therefore unacceptable. Thus, the fiber/primary coating bond desirably should be robust.
Delamination resistance at a given temperature is typically determined by supporting the coated fiber under tension on a support member and driving a cylindrical steel member with a known load against the fiber. After impact, the fiber is observed for delamination, and the test is repeated at another position on the fiber. The load for which 50% of the impacted areas delaminate is referred to as the delamination resistance ("DR50") of the coated fiber. Further details concerning the measurement of delamination resistance are set forth in U.S. Pat. No. 5,908,484 issued to R. L. Decker et al. on Jun. 1, 1999 and entitled "Method of Making A Coated Optical Fiber Comprising Measuring The Delamination Resistance of the Coating", which is incorporated herein by reference.
In what appears to be a requirement contradictory to delamination resistance, it is also desirable to be able to easily strip the primary coating from the glass fiber--particularly when a number of fibers are bonded together in an array such as shown in U.S. Pat. No. 4,900,126. Such an array is frequently referred to as a "ribbon." Indeed, if the coating materials cannot be cleanly and easily stripped, then splicing and interconnecting operations will be seriously hampered.
It is generally believed that the delamination resistance of a coated fiber (and its resistance to stripping) are proportional to the in situ modulus of the primary coating (hereinafter "modulus"). As a consequence, the choice of modulus level has not been to provide maximum protection to the fiber but rather has been constrained to the relatively lower levels associated with easy stripping.